1. Field of the Invention
The present invention relates to a thermal-transfer dye-image-receiving sheet. More particularly, the present invention relates to a thermal-transfer dye-image-receiving sheet capable of producing clear images having a very even color density when used in a dye-image-thermal-transfer printer.
2. Description of the Related Art
Currently, development of a thermal image-transfer printer capable of producing high quality colored images, especially a dye-thermal-transfer printer is progressing rapidly. In the dye-thermal-transfer printer, colored images or pictures are formed by superposing thermally transferred yellow, magenta and cyan colored images or pictures in the form of a number of dots, to produce continuous tone full-color images or pictures having a continuous hue and color density. In a dye-image-thermal-transfer printing operation, an ink sheet composed of a base film and a sublimating yellow, magenta or cyan dye layer formed on the base film is superposed on a dye-image-receiving sheet composed of a substrate sheet and a dye-image-receiving layer formed on the substrate sheet in such a manner that the sublimating dye layer of the ink sheet comes into contact with the dye-image-receiving layer of the dye-image-receiving sheet, and the ink sheet is locally heated by a heat supplied by a thermal head in the printer. The amount of the heat is continuously controlled in accordance with electric signals corresponding to the images or pictures to be printed, and portions of the sublimating dye ink in the dye layer are thermally transferred to the dye-image-receiving layer to provide colored images in a predetermined pattern and having a predetermined color density. It is known that to print high quality colored images on a dye-image-receiving sheet at a high speed using a dye-thermal-transfer printer, a biaxially oriented multilayer film comprising, as a principal component, a thermoplastic resin, for example, polyolefin resins, and having a plurality of voids formed therein, is used as a substrate sheet for the thermal-transfer dye-image-receiving sheet. A conventional thermal-transfer dye-image-receiving sheet, which will be referred to as a dye-image-receiving sheet hereinafter, has a dye-image-receiving layer formed on the above-mentioned substrate sheet and comprises, as a principal component, a dye-receiving thermoplastic resin. Where a biaxially oriented multilayer thermoplastic resin film is used as a substrate sheet, the resultant dye-image-receiving sheet is advantageous in being very even in thickness, in a satisfactory flexibility and softness and in a low thermal conductivity thereof, compared with other dye-image-receiving sheets in which the substrate sheet consists of a conventional paper sheet made from cellulose pulp fibers, and thus the colored images printed on the dye-image-receiving sheet have a high color density and a satisfactory evenness in color density.
Nevertheless, where the biaxially oriented multilayer thermoplastic film is used as a substrate sheet for a dye-image-receiving sheet which is strictly required to exhibit a high reproducability of the colored images, the resultant dye-image-receiving sheet is disadvantageous in that, when the dye images are recorded on the sheet using a thermal head, the remaining stress in the substrate sheet derived from a drawing process applied to the thermoplastic resin film is released, and thus a difference in thermal shrinkage is generated between the substrate sheet and the dye-image-receiving layer to cause the dye-image-receiving sheet to be locally curled and wrinkled. These curls and wrinkles hinder the smooth conveyance of the dye-image-receiving sheet through the printer, and the resultant print has a significantly poor commercial value. Even when the biaxially oriented multilayer film has a front surface layer and a back surface layer equal in thermal shrinking property to each other, since the heat is applied only to the front surface of the dye-image-receiving sheet during the printing operation, the temperature of the front surface is different from that of the back surface, and thus the dye-image-receiving sheet is curled or wrinkled.
Also, the lamination of the biaxially oriented films to form the front and back surface layers causes the resultant multilayer film to be expensive.
To eliminate the above-mentioned disadvantages, for example, the unevenness in color density of the images, by not employing the thermoplastic biaxially oriented multilayer film as a substrate sheet, Japanese Unexamined Patent Publications (Kokai) No. 2-106,397 and No. 2-307,786 disclose a substrate sheet comprising a base sheet consisting of a paper sheet and having a high surface smoothness and a polyolefin coating layer laminated on a base sheet surface and having a thickness of 5 to 35 .mu.m and a basis weight of 5 to 25 g/m.sup.2. In this type of substrate sheet, the quality of the dye image can be improved to a certain extent by controlling the amount of the polyolefin coating layer. However, this type of substrate sheet does not fully eliminate the unevenness in the color density of the received dye images.
U.S. Pat. No. 4,774,224 to Eastman Kodak Co. discloses that to reduce the unevenness in color density and the glossiness of the dye images, the center line surface roughness of a surface of a substrate comprising a substrate paper sheet and an organic polymer layer laminated on the substrate sheet is controlled to a level of 7.5 microinches (7.5.times.25.4.times.10.sup.-3 =0.1905 .mu.m). However, the effect of the laminated substrate in enhancing the evenness in the color density and glossiness of the received dye images is still not satisfactory.